The hemagglutinin protein (HA) of influenza virus has a globular head domain which is highly heterogeneous among flu strains and a stalk region containing a fusion site which is needed for entry into the cells. HA is present as a trimer on the viral envelope. The uncleaved form of hemagglutinin protein (HA0) is activated by cleavage by trypsin into HA1 and HA2 portions to permit the fusion site to effect virulence. The two cleaved portions remain coupled using disulfide bonds but undergo a conformational change in the low pH environment of the host cell endosomal compartment which leads to fusion of the viral and host cell membranes.
The cleavage site contains a consensus sequence that is shared both by influenza A and influenza B and by the various strains of influenza A and B. The uncleaved hemagglutinin protein trimer (HA0) is referred to as the inactivated form, whereas when cleaved into HA1 and HA2 portions, the hemagglutinin protein is referred to as being in the activated form.
Bianchi, E., et al., J. Virol. (2005) 79:7380-7388 describe a “universal” influenza B vaccine based on the consensus sequence of this cleavage site wherein a peptide comprising this site was able to raise antibodies in mice when conjugated to the outer membrane protein complex of Neisseria meningitidis. Monoclonal antibodies which appear to bind to the consensus sequence were also described. In addition, successful passive transfer of antiserum was observed in mice. Other prior art vaccines, such as those described in WO2004/080403 comprising peptides derived from the M2 and/or HA proteins of influenza induce antibodies that are either of weak efficacy or are not effective across strains.
Antibodies described in the art which bind the HA stalk region involve those developed by Crucell, CR6261 and CR8020 described in Throsby, M., et al., PLoS One (2008) 3:e3942, Ekiert, D. C., et al., Science (2011) 333:843-850, and Sui, J., et al., Nat. Struct. Mol. Biol. (2009) 16:265-273. An MAB has also been developed against the conserved M2E antigen as described by Grandea, A. G., et al., PNAS USA (2010) 107:12658-12663. M2E is on the surface of infected cells and is also the target of amantadine and rimantadine. Drug resistance has occurred against these antibiotics which suggests that this target does not serve an essential function.
An additional antibody has been described by the Lanzavecchia Group: Corti, D., et al., Science (2011) 333:850-856 which binds and neutralizes both Group 1 and Group 2 strains of influenza A, but the potency is not as high as those described herein as shown in the examples below. In addition, an MAB that is immunoreactive against both influenza A and B as described in Dreyfus, C., et al., Science (2012) 337:1343-1348 has less potency than those described below.
PCT application publication No. WO2011/160083, incorporated herein by reference, describes monoclonal antibodies that are derived from human cells and useful in passive vaccines. The antibodies show high affinities of binding to influenza viral clade H1, which is in Group 1, and some of the antibodies also show high affinities to H9, also in Group 1 and/or to H7 in Group 2 and/or H2 in Group 1. Some of the antibodies disclosed bind only the inactivated trimer form, presumably at the consensus cleavage region, while others are able to bind activated hemagglutinin protein which has already been cleaved.
There remains a need for antibodies that bind additional clades and show enhanced affinity thereto.